The present invention relates generally to the reading of the particular contour of the rims of any spectacle frame for the purpose of cutting ophthalmic lenses for mounting in those rims, this reading of the contour permitting either direct cutting of lenses in a machine for trimming and bevelling ophthalmic lenses, or indirect cutting by making reproduction templates which can then be mounted in such a machine.
Several particular problems have to be overcome in contour reading in this manner in order to ensure that the contour reading will subsequently permit the cutting of an ophthalmic lens whose contour corresponds as accurately as possible to the contour of the rim in which the lens has to be mounted.
Firstly, it is desirable to follow the bottom of the bezel provided in the rim which is being read, because the contour of this bezel actually determines the peripheral contour of the ophthalmic lens to be mounted in the rim.
In addition, the "meniscus" effect usually present in a rim of this kind must also be taken into account, that is to say the curvature of the surface on which the rim can as a whole be inscribed.
Finally, it is desirable that the contour should be read by reference to the plane tangential to this contour, in such a manner as to follow any variation of curvature thereof as closely as possible, particularly in the zones where these variations of curvature are greatest.
The expression "tangential plane" means a plane which contains the tangent to the point in question on the contour of the bezel of the rim being read, and which is perpendicular to the medium plane of this rim, which is assumed to be plane, and therefore parallel to the optical axis of the ophthalmic lens which is to be mounted in the rim.
Contour readers proposed hitherto for reading the contour of spectacle rims do not satisfactorily comply with the requirements briefly set forth above.
Generally contour readers of this kind comprise one or more feelers with which the rim of the spectacle frame whose contour is to be read must be kept permanently in contact, these readers being additionally provided for this purpose with a support plate to which is fastened in appropriate manner a spectacle frame whose rim contour has to be read, this support plate being mounted for translational movement by displacement control means in order to apply the rim being read against the feeler or feelers for rotation about the geometrical axis of the said rim, and for rotation by pivoting about the corresponding point of contact, while a second support plate synchronised with the first-mentioned plate also carries, facing a cutting tool, either the template or the ophthalmic lens which is to be cut.
In a known apparatus of this kind there are a plurality of feelers, actually cams, each of which has an arm extending parallel to the optical axis of the rim being read, and in contact with the inside contour of the said rim.
Such feelers or cams are not able to follow the contour of the bezel of the rim which they are reading, neither do they take into account the meniscus effect of a rim of this kind.
In another known countour reader the feeler is a rod provided with a reading head which is adapted to co-operate with the bezel of the spectacle frame rim.
In this reader however, although the rod carrying the reading head is mounted for axial sliding in its support, no particular arrangements are made for taking into account the meniscus effect of the rim being read; on the contrary this rod is locked axially in its support during reading.
In yet another known reader the feeler is also in the form of a rod which is provided with a reading head which is articulated on the rod about an axis perpendicular to the axis of the rod.
This arrangement actually enables the reading head to follow correctly the bottom of the bezel of the rim of the spectacle frame being read, despite the meniscus effect of the rim. However the precise modifications of angular orientation of this reading head relative to the rod which carries it in order to enable the head to follow the bezel taking into account the meniscus effect of the bezel, cannot fail to give a false reading.
Furthermore, the contour readers briefly described above have the common disadvantage of lack of sensitivity in relation to the tangential plane at the reading point, as defined above, and therefore do not permit accurate analysis of the contour read, particularly in zones where the radius of curvature of this contour varies rapidly.
In order to overcome this disadvantage it has also been proposed to use two feelers simultaneously which feelers are at a distance from each other.
This provides better sensitivity in relation to the tangential plane at the point of reading of the contour, but it results in relative complexity of the apparatus. In addition the apparatus does not take into account the meniscus effect of the contour which is to be read.
It is a main object of the present invention to permit the reading of a contour, and in particular the reading of the contour of the bezel of a spectacle frame rim, which provides satisfactorily the requirements briefly explained above, namely the requirement of taking into account the meniscus effect which may be present in the contour which is to be read, and the requirement of sensitivity to the tangential plane at the point of reading of this contour.